The present invention relates to a system and method for removing pollution from water.
It has long been desirable to remove pollutants from water in a safe, efficient and cost-effective manner. Agricultural, industrial, and street runoff, among other polluted water flows, require treatment before being released into the environment. The high concentration of pollutants in these wastes can overwhelm self-purifying mechanisms in the receiving environment. When this occurs, the result is contaminated ground water and/or surface water.
Public wastewater treatment systems typically serve high-density population areas. However, in less densely populated areas or where public sewage treatment is not available, many homes and businesses use a septic system, implemented on-site, for the treatment and disposal of wastewater or sewage.
A typical on-site waste fluid treatment, or septic, system includes a mound or drainfield and a septic tank. Waste fluids such as household sewage may include wastewater from washers and dryers, showers and bathtubs, toilets, disposals, disposal waste, sink wastewater and wastes from various commercial operations. In a typical on-site septic system, the wastewater generally drains into a septic tank before being routed to the mound or drainfield. A septic tank typically separates sewage into solid and liquid fractions, then introduces the separated liquid fraction (effluent) back into the ecosystem with significant levels of undesirable nutrients and other pollution. Undesirable compounds present in the effluent are then decomposed and utilized by soil organisms in the drainfield or mound as the effluent moves (percolates) through the underlying soil profile.
The average life of a conventional on-site wastewater treatment (septic tank and drainfield or mound) system is typically only seven to ten years. A failing septic system can lead to public health concerns and non-point source pollution. Another related concern is the difficulty of quickly and accurately assessing whether the underground septic system is functioning properly. If the conventional system is not functioning properly, untreated, hence polluting, wastewater is likely being released into the ecosystem with little or no surface indication.
A primary concern with any on-site septic system or wastewater treatment system is to ensure that nutrients and other pollutants are removed from the wastewater before the wastewater enters a surface or subterranean body of water. If the treated wastewater is not sufficiently pollutant-free, the effluent will create water quality problems by contaminating surface or subsurface bodies of water.
The ability of wetland plants to remove pollutants from wastewater is known. To this end, natural wetlands have been used as wastewater discharge sites for a long period of time. Thus far, however, constructed wetlands have made only limited use of the potential of wetland vegetation to purify (detoxify) wastewater.
Existing constructed wetlands, including both surface and subsurface flow systems, utilize only wetland plants and atmospheric diffusion to transfer oxygen into (oxygenate) the wastewater being treated (the water column). These naturally aerated (aerobic) zones support populations of oxygen-requiring bacteria. Other areas within the constructed wetland, which are not oxygenated, are anaerobic and support populations of bacteria which do not require oxygen. It is known that aerobic metabolic pathways are much more efficient than anaerobic pathways in decomposing certain types of pollutants. Consequently, aerobic bacteria are capable of consuming, and thus removing, more pollutants than anaerobic bacteria for a given treatment cell size.
In existing constructed wetlands, aerobic zones are typically found only at the top of the water column and in the immediate vicinity of wetland plant roots. The top of the water column is usually a region where there is sufficient gas exchange, via atmospheric diffusion. In the immediate vicinity of wetland plant root hairs, oxygenxe2x80x94translocated by wetland plants into their root systemsxe2x80x94diffuses out through the root membranes. These naturally occurring aerobic zones occupy only a small portion of the wetland liquid volume. Thus, the ratio of aerobic activity to anaerobic activity is usually extremely small in natural wetland systems. This lack of aerobic capacity thus limits the overall treatment capacity of the wetland, particularly in subsurface flow constructed wetlands.
FIG. 1 depicts a conventional on-site septic system, and FIG. 2 depicts a constructed wetland treatment system. In FIG. 1, the conventional, on-site septic system is depicted generally at 50. The conventional system 50 includes a sewer line 52 conveying sewage from a house 54 to a septic tank 56. In the septic tank 56, the solids are allowed to settle out of the sewage. The separated liquid wastewater effluent flows from the septic tank 56 to the drainfield (or mound) 58 via a sewer line 59. In the septic tank 56, the wastewater is treated to a limited extent when compounds present in the settled solids and effluent undergo predominantly anaerobic decomposition. However, levels of pollutants present in the wastewater being conveyed from the septic tank 56 are usually too high for direct release into the environment (e.g., direct release into a body of water such as a stream, lake, or aquifer). The final disposal and treatment of wastewater occurs in the drainfield 58, which includes a series of perforated pipes 60. Thus, the wastewater is conveyed by the sewer line 59 from the septic tank 56 to perforated pipes 60 within the drainfield 58. The partially treated wastewater seeps from the perforated pipes 60 into the soil profile underlying the drainfield 58. In the underlying soil profile, the wastewater effluent undergoes a final series of purification steps as it percolates predominately down as discharge 62 through the soil profile.
These purification steps are accomplished by soil organismsxe2x80x94mostly soil microflora. Thus, whether the wastewater effluent will ultimately be purified to an acceptable level before entering a body of water depends on the ability of the soil profile to accommodate the liquid flow and to harbor soil microflora. The texture of the soil profile must permit the wastewater to enter the soil profile from the perforated pipes 60 and percolate generally downwardly, e.g., without pooling the wastewater. Soils with high levels of clay or organic matter generally have low capacities to hydraulically convey wastewater in this respect. Coarse textured soils have higher proportions of sand and silt particles and possess higher degrees of hydraulic conductivity. The discharged wastewater (discharge 62), whether or not adequately treated, percolates down to aquifers or can also be conveyed somewhat laterally into exposed bodies of water 64. Thus, one limitation of the conventional system depicted in FIG. 1 is that the drainfield 58 cannot be located within a specified distance from to a surface body of water or cannot be used at all if the water table (aquifer) underlying the drainfield or mound is sufficiently high. Another disadvantage of the system 50 is that the soil profile underlying the drainfield 58 will slowly lose its hydraulic conductivity. The loss of hydraulic conductivity is due to such factors as unsettled solids conveyed by the wastewater, soil bacteria, substances associated with soil bacteria (e.g., polysaccharides), and solidified wastewater components. These substances slowly fill the spaces (pores) between the soil particles. When these pores become filled, the soil becomes incapable of conducting the wastewater through the soil profile and the wastewater cannot be exposed to the soil bacteria. In time, the conventional disposal system 50 will fail to purify the sewage and will itself become a source of pollution. Present remedies for failed disposal systems of this nature include replacing and/or relocating some or all of the components (e.g., the drainfield), xe2x80x9crestingxe2x80x9d the system by discontinuing disposal system use for a period of time, and treating the soil with chemical agents. Discontinuing use of the disposal system is often not feasible. Treating the soil with chemical agents has not been effective in most situations. Thus, the expensive process of replacing and/or relocating underground components is often the only feasible method of restoring a functional septic system.
FIG. 2 depicts a sewage disposal system 70. The sewage disposal system 70 is similar in concept to the conventional system 50 of FIG. 1, but includes a wetland cell 72 and a chemical absorption tank 74. In the system 70, sewage from the house 54 is conveyed to the septic tank 56 via the sewer line 52. In the septic tank 56, solids settle out of the sewage and the separated wastewater effluent is then conveyed from the septic tank 56 to the wetland cell 72 by line 59. While the wastewater is present in the wetland cell 72, many of the pollutants therein are decomposed by mostly anaerobic microflora and wetland vegetation 76. The wetland vegetation 76 also removes some of the inorganic pollutants (e.g., nitrates, phosphates, potassium) as plant nutrients. From the wetland cell 72, the partially treated wastewater is then conveyed to the chemical absorption tank 74. The chemical absorption tank 74 usually contains minerals such as limestone or other substances (e.g., activated charcoal, kiln-fired clay beads, wollastonite, taconite tailings) with high surface areas and absorptive characteristics to further remove undesirable compounds from the wastewater being treated. The fluids are then discharged from the chemical absorption tank 74 directly into the environment if pollutant levels are within acceptable limits. Alternatively, the fluids from the chemical absorption tank 74 may be conveyed to a drainfield 58 as described above for further purification by soil microflora.
Because of the low amount of aerobic habitat present, the disposal system 70 has a disadvantage of relying primarily on anaerobic microflora in the septic tank 56 and the wetland cell 72 to decompose undesirable compounds. Another disadvantage of the disposal system 70 is that the wetland cell 72 is exposed to the atmosphere and, hence, subject to being frozen during winter months. When frozen, the entire system 70 becomes inoperative. Moreover, the roots of the wetland vegetation 76 may be injured or entirely killed if sufficiently low temperatures occur for significant periods of time.
Accordingly, it would be desirable to more fully utilize the pollution and nutrient-reducing characteristics of wetland plants in a constructed system to treat polluted water, the system incorporating a better efficiency of increased aerobic microbial habitat and preferably remaining operational during lower winter temperatures.
The present invention provides a safe, efficient, and cost-effective manner of reducing pollutant levels in water or other fluids.
One preferred embodiment of this invention includes a substantially impermeable primary treatment cell and an optional secondary treatment cell. The primary treatment cell includes a forced aeration system. A fluid level control system, such as a dosing siphon, may be in fluid communication with the primary treatment cell and the secondary treatment cell (if the secondary treatment cell is present). The substantially impermeable primary treatment cell includes a bed medium such as gravel, a mulch layer, wetland vegetation rooted in the bed medium and extending through the mulch layer, and a forced aeration system. The secondary treatment cell may be substantially permeable to allow treated wastewater within to egress by infiltration and may further contain a bed medium for further removal of pollutants. The dosing siphon lowers the level of the wastewater being treated in the primary treatment cell so that lower wastewater levels are present for a sufficient amount of time to stimulate deeper, more pervasive root growth within the bed medium. The lower water levels also provide atmospheric oxygen to, and thereby stimulate the growth of, aerobic bacteria. The forced aeration system establishes alternating aerobic and anaerobic zones within the substantially horizontally-flowing wastewater being treated in the primary treatment cell so that wastewater in the substantially horizontal flow is exposed to aerobic and anaerobic zones for a significant period of time. By being exposed to both aerobic and anaerobic zones for a significant period of time, the decomposition of pollutant compounds occurs more rapidly and completely than if predominantly anaerobic zones were present.
It is an object of this invention to provide a constructed subsurface flow, wetland system that can be used efficiently, effectively and safely to remove pollutants from wastewater.
It is a further object of this invention to provide for a calculated variable water level management of wastewater in the constructed wetland subsurface treatment system to promote faster establishment of wetland vegetation, to promote thicker and deeper root growth of the wetland vegetation, and to thereby promote more effective pollutant removal processes.
It is yet another object of this invention to provide a substantially impermeable constructed wetland cell for treating wastewater which is characterized by a generally vertical and unsaturated wastewater flow in a preferred embodiment. The wetland cell may include a bed medium, a wastewater supply system, a wastewater return system, a forced aeration system, and a multiplicity of plants. The wastewater supply system may be configured to deliver the wastewater proximate an upper portion of the bed medium. At least a portion of the wastewater return system may be disposed proximate a lower portion of the bed medium. The multiplicity of plants may be rooted in the bed medium. In this constructed wetland cell, the wastewater flows generally vertically downward from the wastewater supply system, through at least a portion of the bed medium, and is conveyed from the wetland cell via the wastewater return system. The wastewater supply system, the wastewater return system, and the bed medium may be further be configured and disposed such that an unsaturated flow conveys the wastewater from the wastewater supply system, through at least a portion of the bed medium, to the wastewater return system. The wetland cell may further include an air source for increasing the oxygen supply within the constructed wetland cell. A forced aeration (air supply) system may be present and may include a blower and pipes with perforations. The perforated pipes may be disposed proximate a bottom portion of the bed medium and the blower may force atmospheric air through the pipe perforations such that the wastewater becomes oxygenated while flowing through spaces in the bed medium. The wetland cell may further include a layer of substantially decomposed mulch overlaying the bed medium. The substantially decomposed mulch may comprise peat.
It is a further object of this invention to provide a system for treating wastewater, the system including a forced aeration (air supply) system, a constructed wetland cell, and a disposal system. The air supply may be configured to increase oxygen concentration in the wastewater being treated by aspirating air into the wastewater. The constructed wetland cell may include a bed medium, a wastewater supply system, a wastewater return system, and a multiplicity of plants. At least a portion of the wastewater supply system may be disposed proximate an upper portion of the bed medium. At least a portion of the wastewater return system may be disposed proximate a lower portion of the bed medium. The plants may be rooted in the bed medium and extend through the mulch layer. The disposal system may receive treated wastewater from the constructed wetland cell. The system may further include a substantially decomposed mulch layer (e.g., peat) overlaying the bed medium. The system may yet further include a structure with a chamber for separating solids from the wastewater, such as a septic tank. A substantial portion of the solids are ideally separated from the wastewater before the wastewater is conveyed to the constructed wetland cell. The system may still further include a filter tank receiving wastewater from the septic tank and treated wastewater from the constructed wetland cell. The wastewater from the septic tank and the treated wastewater from the constructed wetland cell are blended in the filter tank. The system may still yet further include a recirculation tank receiving the filtered and blended wastewater from the filter tank and conveying the filtered and blended wastewater to the constructed wetland cell. The system may still yet further include a dosing tank receiving treated wastewater from the recirculation tank. The dosing tank may convey the treated wastewater to a disposal system. Alternatively, the treated wastewater may be conveyed from the recirculation tank directly to the disposal system.
Another embodiment of the present invention includes a structure with a cavity for settling solids from sewage and a recirculation chamber in fluid communication with said cavity. A pump and aspirator are present. The pump pumps the wastewater from the recirculation chamber through the aspirator, thereby increasing the dissolved oxygen concentration in the wastewater. From the aspirator the wastewater is conveyed to a substantially impermeable wetland cell. The wetland cell may include a bed medium, a wastewater supply system and a wastewater return system. At least a portion of the wastewater supply system is disposed proximate an upper portion of the bed medium and at least a portion of the wastewater return system is disposed proximate a lower portion of the bed medium. The oxygenated wastewater is pumped from the pump and aspirator, through the wastewater supply system, to an upper portion of the bed medium and allowed to flow substantially vertically through the bed medium. When the wastewater arrives at a lower portion of the bed medium, it is conveyed away from the wetland cell by the wastewater return system to the recirculation chamber. In the recirculation chamber, treated wastewater from the wetland cell is blended with untreated wastewater from the settling cavity. The blended wastewater may be cyclically routed to the wetland cell or conveyed to a disposal system.
Yet another embodiment of the present invention includes a structure with a wetland unit and an anaerobic, fluid-impermeable unit, optionally integrally formed in an easily installed unit. The wetland unit includes a granular bed medium. A mulch layer is optionally present overlaying the medium and vegetation is optionally rooted in the medium. The anaerobic unit is optionally positioned beneath the wetland unit and may form first and second chambers. The first chamber accommodates an inlet, a mixing device and a filter. The mixing device may possess a large surface area to provide habitat for anaerobic microflora. The filter removes particulates from the wastewater as the wastewater is conveyed from the first chamber to the second chamber. A pump may be operationally present in the second chamber to convey wastewater to the wetland unit. The pump may be considered as part of a wastewater supply system. An aerator is optionally present in the conduit between the pump and the wetland unit. The pump also conveys wastewater to an outlet. Operationally, the pump conveys wastewater, via the wastewater supply system to an upper portion of the bed medium. If the aerator is present the wastewater is aerated when being pumped from the second chamber to the wetland unit. After being conveyed from the wastewater supply system, the wastewater, in a substantially vertical and unsaturated flow, flows through the bed medium to a lower portion thereof, where it is then conveyed to the mixing device in the first chamber. The wastewater is aerated while flowing through the bed medium and is aerobically treated during the substantially vertical, unsaturated flow through the bed medium. The mixing device is positioned to receive exogenous wastewater from the inlet and treated wastewater from the wetland unit and to mix the two wastewater flows to enable anaerobic or anoxic decomposition of wastewater pollutants in the first and second chambers. The wastewater is cyclically conveyed: from the first chamber to the second chamber, from the second chamber to the wetland unit, and from the wetland unit to the first chamber. In the first chamber, the treated wastewater from the wetland unit is mixed with exogenous wastewater. The wastewater is also pumped from the second chamber to an outlet when the wastewater has been sufficiently treated. Alternatively, the wastewater is pumped from the wetland unit to the outlet without being mixed with exogenous wastewater.
These and other objects, features, and advantages of this invention will become apparent from the description, which follows and when considered in view of the accompanying figures.